The present invention relates to a method for producing oligosaccharides which are growth factors for bifidobacteria in vivo.
In particular, the present invention has been achieved with a view to obtaining high-purity oligosaccharides containing less unreacted lactose and by produced monosaccharides, such as galactose and glucose, in a production of oligosaccharides by reacting lactose with .beta.-galactosidase from Aspergillus oryzae.
Oligosaccharides expressed by the formula Gal--(Gal).sub.n --Glc (wherein Gal denotes a galactose residue; Glc, a glucose residue; and n, an integer of 1 to 4) (simply referred to as "oligosaccharides" hereinafter) are major components of oligosaccharides in breast milk, and are useful as growth factors for bifidobacteria which are useful bacteria in the human intestines.
In the method of "oligosaccharides" production, it is a typical method that lactose is reacted with .beta.-galactosidase from Aspergillus oryzae (published in Japanese Patent Publication No. 58-20266). In this method, the following condition is recommended: reaction temperature of 20.degree. to 50.degree. C., a concentration of lactose of about 10 to 50%, a pH of 3 to 8, and a concentration of enzyme of 1 to 100 units/ml. The above-described upper limit of the concentration of lactose is related to the optimum temperature of .beta.-galactosidase from Aspergillus oryzae. Because, lactose is difficult to dissolve much in water, so only about 50% of it can be dissolved in water within the range of optimum temperatures of the .beta.-galactosidase from Aspergillus oryzae.
The above-described method of producing "oligosaccharides" which uses .beta.-galactosidase from Aspergillus oryzae is excellent in view of the fact that it can provide a higher yield of oligosaccharides than methods using .beta.-galactosidase from other microorganisms. But it has a problem in that the maximum ratio of the conversion of lactose into "oligosaccharides" is only about 27%. In addition, it is difficult to separate the "oligosaccharides" from the reaction product which contain large amounts of unreacted lactose and by produced monosaccharides such as galactose and glucose. Therefore, when using "oligosaccharides" the above-described reaction products are normally used as they are. As a result, unnecessary lactose and monosaccharides are often used together with the "oligosaccharides", resulting in the problems that foods and drinks containing the "oligosaccharides" have high calorie levels and are difficult for patients with lactose intolerance to uptake. Therefore, there has been a disadvantage in that the usable range and usages of such products are limited.
The above-described method also has a following problem. Since the sugar(s) concentration of the reaction solution is low, it is necessary to concentrate for preventing contamination with bacteria and reducing the product transportation costs, and thus large heat-energy costs and equipment costs are necessary.
It is an object of the present invention to improve the above-described old method of producing "oligosaccharides" using .beta.-galactosidase.